In a production line, whether a workpiece (a detection target object) exists is determined based on a physical amount such as a light reception amount detected by a sensor. The physical amount is detected at a plurality of points. That is, the plurality of sensors are provided. At this point, for example, a light receiving unit of a photoelectric sensor is occasionally disposed near a light receiving unit of another photoelectric sensor. In this case, light projected from a certain photoelectric sensor is incident to a light receiving unit of another photoelectric sensor to degrade determination accuracy. Therefore, there is provided a technique of preventing mutual interference by varying an operation time of the sensor.
Japanese Patent No. 3478646 discloses a technique of varying a light projecting time of the photoelectric sensor. In Japanese Patent No. 3478646, a master unit outputs an external synchronous signal for a predetermined period in a self-excitation manner. When receiving the external synchronous signal, a slave unit generates a light projection pulse signal at a time delayed by a predetermined time compared with the external synchronous signal, and externally outputs the external synchronous signal.
However, in Japanese Patent No. 3478646, because the synchronous signal is output when the slave unit projects the light, the light projecting time of the slave unit located at a lower stage depends on the light projecting time of the slave unit located at an upper stage. For example, when the slave unit located at the upper stage is the photoelectric sensor of a type (a model) having the long light projecting period, because the light projecting period becomes longer than a period during which the synchronous signal is transmitted to the slave unit located at the lower stage, the light projecting period of the slave unit located at lower stage is also lengthened. Accordingly, the light projecting period of the photoelectric sensor located at the lower stage is unnecessarily lengthened in a system in which the photoelectric sensor of the type having the short light projecting period and the photoelectric sensor of the type having the long light projecting period are mixed. Therefore, possibly a response speed is decreased to degrade measurement accuracy.